The Kinetic Theory

The Particle Model (Year 9) and the Collision Theory (Year11) are both important in understanding why reactions and phase changes occur. We are about to learn about factor that control the rate of reaction, but first we had to revisit these key ideas from previous years:

Bond Enthalpy

Bond enthalpy is the amount of energy required to break one mole of a particular bond. As bond breaking requires energy, these values are always exothermic.

Calculating Enthalpy Change

We do experiments using calorimeters to find enthalpy changes. The change in temperature of the solution is translated into an energy change for the water component of this solution. This is then inferred to be the equal (but opposite) value for the energy change for the reactants. This can then be used to calculate the enthalpy change.

Enthalpy Change Diagrams

We were asked to do the same for an endothermic reaction. For homework, we have to read pp140-142 and attempt #2, p144 in Beginning Chemistry.

Molecular Polarity

The last part of this unit is about molecular polarity. We combine the ideas of bond polarity and shape to predict and justify molecular polarity.

Here is the blog post etc. from last year:

Shapes of Molecules

Here is the video of me teaching it to his class last year:

There is also an excellent series of lessons in Khan Academy worth working through: HERE

I really like this guy's video too:

In class, we were also asked to work on a past NCEA question: Discuss the shapes and bond angles in NOCl and H₂S (the Lewis Dot Diagrams were provided in the question):

The Lewis diagram for NOCl shows there are three regions of electron density around the central atom (N). This means the electron cloud has a trigonal planar shape with bond angles of 120°. However, as there are only two atoms bound to the nitrogen atom (O & Cl), this is a bent molecule. Due to the repulsion of the lone pair of electrons on the nitrogen atom, the bond angle would be slightly lower than 120°.

The Lewis diagram for H₂S shows there are four regions of electron density around the central atom (S). This means the electron cloud has a tetrehedral shape with bond angles of 109.5°. However, as there are only two atoms bound to the sulfur atom (H in both cases) this is a bent molecule. Due to the repulsion of the two lone pairs of electrons on the sulfur atom, the bond angle would be slightly lower than 109.5°.

2013 Blog

While we have a Student-Teacher, it may be worth looking at the 2013 Level 2 Chemistry blog for additional videos etc. This topic was taught in April and May in 2013:

http://l2chem2013.blogspot.co.nz/